heater is blowing cold air when on hot have replaced radiator and flushed? on 2000 Ford Expedition

what could cause this and do you have a D.I.Y. solution to this this a common problem

Asked by for the 2000 Ford Expedition
also ck coolant level
2 more answers
also ck coolant level
Check the heater to ensure the trash you just flushed didn't go into the core and stop it up as well. Next would be to check/test the temp/blend door actuator.